The move is a response to the UN Security Council statement condemning North Korea's missile launch last week.
It threatened Wednesday to shut its border with South Korea, where activists have been launching giant balloons carrying leaflets.
The Yongbyon reactor could be operational again within a year.
A US diplomat returns from talks in Pyongyang Monday with 18,000 pages supplied by North Korea on its nuclear facilities.
President Lee can lean on his new Assembly majority to pursue economic reforms and a tougher N. Korea policy.
Missile launches and the expulsion of South Korean personnel from a joint industrial zone show Pyongyang's irritation with Seoul.